I think the detail section of all mines (e.g. iron mines, clay pits, etc.) reports the state of the workers wrong. Although the workers are ‘gathering ore’ most of the time, the state shows them as ‘transferring goods’ instead of ‘working’.
E.g. in this screenshot the detail shows “74% Transferring Goods” and “0% Working”, although the workers are actually producing clay.
It’s the same for coal and iron mines.
e: this is in the current version, 0.9.0p8